What is the best or fast way to add a star on top of a helmet?
I would suggest you use a shinkwrap modifier, as well as a solidify modifier to give it some thickness, and possibly a bevel:



This applies to any shape you'd like to project onto any surface.

Just keep in mind that you need to have enough geometry to allow it to deform smoothly, and preferably a quad loop around the border to avoid shading (or bevel) artefacts: